Fuel cells electrochemically convert fuels and oxidants to electricity, and fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as a basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In the typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. As is known in the art, some of the fluid manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates for each fuel cell. Also, other fluid manifolds circulate coolant (e.g., water) for cooling.
As is known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. So, the fuel and oxidant fed to each fuel cell are usually humidified. Furthermore, a cooling mechanism is commonly employed for removal of heat generated during operation of the fuel cells.
Flow field plates are commonly produced by any of a variety of processes. One plate construction technique, which may be referred to as "monolithic" style, compresses carbon powder into a coherent mass. Next, the coherent mass is subjected to high temperature processes which bind the carbon particles together, and convert a portion of the mass into graphite for improved electrical conductivity. Then, the mass is cut into slices, which are formed into the flow field plates. Usually, each flow field plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. Typically, flow field channels are engraved or milled into a face of the rigid, resinimpregnated graphite plate. In order to effectively distribute reactant fluid and/or humidification fluid for the PEM, it is desirable that the flow channels remain as open and unclogged as possible.
An exemplary configuration for fuel cell membrane hydration and fluid metering is disclosed in U.S. application Ser. No. 08/899,262 by Jones and Walsh (entitled "Fuel Cell Membrane Hydration and Fluid Metering," filed Jul. 23, 1997, and assigned to Plug Power, L.L.C.), which is hereby incorporated herein by reference in its entirety. In one aspect, a bridge or cover plate can extend along the face of a fluid flow plate and across the inlets thereof, defining one opening or injection port for each inlet in addition to an input orifice to a fluid manifold. The inlets can receive respective portions of a given stream of reactant fluid for the fuel cell. Each injection port can inject a portion of liquid water directly into its respective flow channel in order to mix its respective portion of liquid water with the corresponding portion of the stream. This serves to hydrate at least corresponding parts of a given membrane of the corresponding fuel cell. The hydration system may be augmented by a metering system, including flow regulators. Each flow regulator can meter an injecting of liquid portions at the plate inlets into a given fluid stream. The bridge or cover plate may be interposed between a gasket and the fluid flow plate, at the inlets thereof.
However, it remains desirable to provide refinements to such membrane hydration and fluid metering as well as further enhancements for fluid service, including enhanced flow plate configuration and fluid manifolding. For instance, it may be desired to eliminate need for a bridge or cover plate, while still offering advantages thereof. For example, it is advantageous to protect inlets for flow channels on a fluid flow plate from intrusion by an adjacent material, such as a gasket. Furthermore, it is desirable to maintain a clamping pressure on a membrane electrode assembly, such as between gaskets adjacent each fluid flow face of a fuel cell. Namely, one would wish to avoid reactant fluid (e.g., gas) on one side of the fuel cell from leaking around an edge of the membrane electrode assembly into the opposite side of the fuel cell. In particular, an insufficient clamping of the membrane electrode assembly may allow flapping thereof in the presence of pressurized reactant fluid flow, which may cause leakage of the reactant fluid at the plate inlets to an opposite side of the fuel cell, with deleterious consequences (e.g., explosion).
Thus, a need exists for an improved mechanism for maintaining open and unclogged, flow channels of a fluid flow plate in a fuel cell assembly. A further need exists for such a mechanism to promote stability and support for the fuel cell assembly. An additional need exists for fluid flow plates which are shaped to optimize fluid service. A still further need exists for reducing the number of parts for a fuel cell stack, including easing the assembly thereof. Yet another need exists for coordination and cooperation among fluid flow plates in increasing design flexibility, and in providing features for enhanced fluid service and fuel cell assembly operation.